Chemical mechanical polishing (CMP) has been successfully used for planarizing both metal and dielectric films. In one plausible mechanism of planarizing, the polishing process is thought to involve intimate contact between high points on the wafer surface and the pad material, in the presence of slurry. In this scenario, corroded materials, produced from reactions between the slurry and wafer surface being polished, are removed by shearing at the pad-wafer interface. The elastic properties of pad material significantly influence the final planarity and polishing rate. In turn, the elastic properties are a function of both the intrinsic polymer and its foamed structure.
Historically, polyurethane-based pads have been used for CMP because of their high strength, hardness, modulus and high elongation at break. While such pads can achieve both good uniformity and efficient topography reduction, their ability to rapidly and uniformly remove surface materials drops off rapidly as a function of use. The drop off in material removal rates as a function of time observed for polyurethane-based pads has been attributed to changes in the mechanical response of such polishing pads under conditions of critical shear. It is generally believed that the loss in functionality of polyurethane-based CMP pads is due to pad decomposition from the interaction between the pad and the slurries used in the polishing.
Moreover, decomposition produces a surface modification in and of itself in the case of the polyurethane pads which can be detrimental to uniform polishing. Alternatively, in some instances, the surface modification of materials used for CMP polishing pads may improve the application performance. Such modifications, however are temporary, thus requiring frequency replacement or retreatment of the CMP pad. Polyurethane pads also generally require a break-in period before polishing, in addition to reconditioning and pretreatment after a period of use. It is often also necessary to keep traditional pads wet while polishing equipment is in idle mode. All of these characteristics undesirably reduce the overall efficiency of CMP when using polyurethane or similar conventional pads.
Accordingly, what is needed is an improved CMP pad capable of providing a highly planar surface during CMP and having improved longevity, while not experiencing the above-mentioned problems.